Use a ground lift box between the focusrite stereo outs and the amplifier/mixer/whatever comes first in your PA system.
You have a loop like so:
house power > power cable > computer > focusrite > PA system > PA power > house power
If you can't do a ground lift, use batteries.
The swirling sound is capacitor flux (that sounds more complex than it is
), changing the frequency of discharge which you are hearing because the noise/tones generated in a ground loop are the result of an electrical signal being converted to sound by speakers.
Another thing that may work but is expensive is to purchase an isolation transformer box for the wall plug. Power either the computer or the PA from that and you've broken the loop.